Fort Belan

Fort Belan is a coastal fortress in North Wales. It is located opposite Abermenai Point, at the south-western end of the Menai Strait, on the coast of Gwynedd, in the parish of Llanwnda. Situated at the tip of the Dinlle Peninsula, the windblown, north-westernmost point of the Welsh mainland, the fort is cut off twice a day by the incoming tide.

Source: Wikipedia